net.*
Usenet's original flat namespace: for most of the 1980s, every newsgroup of general interest was called net.something.
The prefix separated what was meant to travel the whole network from local groups that stayed on one machine or campus. After it came the topic, with no further structure: net.general, net.jokes, net.unix-wizards, net.flame. With a few dozen groups that was fine. With several hundred, no administrator could tell which groups were heavy, serious or merely expensive to pass along.
The Great Renaming of 1986–87 sorted the groups into seven topical hierarchies, so net.jokes became rec.humor and the technical material moved under comp. The migration took months, and plenty of groups existed under both names in the meantime. Today the prefix survives mainly in old archives, where it is a reliable sign that a post predates 1987.